Do you get a failure code? Did you try it a few time and always the same outcome? Is date and time correct? Is your Ms account synced? Can you open your OneDrive app on your phone? Can you download apps from the store? Did you try a soft reset? If auto backup is on and you do not see a backup than it has never worked so maybe a "setting" problem. How long do you have your phone?

Do you get a failure code? Did you try it a few time and always the same outcome? Is date and time correct? Is your Ms account synced? Can you open your OneDrive app on your phone? Can you download apps from the store? Did you try a soft reset? If auto backup is on and you do not see a backup than it has never worked so maybe a "setting" problem. How long do you have your phone?

No failure code only a backup failed. All the other questions answers are" yes" and I got the phone for almost 3 months. And I'm not a developer of that matters

I am nearly out of ideas. Maybe an app ( in the background ) prevents the backup. Bad wifi/data connection level. If you use cellular data check Data Sense maybe you find some setting change can help. Do you still have space on one drive. ????